1. Field of the Invention
The present invention relates in general to a bound stopper installed on a suspension system of a motor vehicle, which stopper is fitted on a piston rod of a shock absorber, and more particularly to such a bound stopper of a composite type which exhibits a nonlinear spring characteristic.
2. Discussion of the Prior Art
In one type of vehicle suspension for supporting the vehicle body via a shock absorber, there is used a buffer or cushioning member generally called "bound stopper" (also called "bumper spring" or "bound bumper"), for elastically limiting an excessive amount of stroke of the shock absorber. The bound stopper is a generally cylindrical member formed of an elastic material, and is fitted on a piston rod of the shock absorber and thus installed on the suspension. This bound stopper is adapted to abut on a cylinder of the shock absorber upon oscillating movement of the piston rod, so as to elastically limit the amount of stroke of the piston rod.
The bound stopper as described above is desirable to have a so-called nonlinear spring characteristic. Namely, the bound stopper is required to exhibit a soft spring characteristic when the stroke of the shock absorber is relatively small, so as to assure good driving comfort, and exhibits a hard spring characteristic when the stroke is relatively large, so as to assure good driving stability and controllability.
However, it is difficult for the conventional bound stopper formed of a rubber material to provide a sufficiently reduced spring constant when the stroke is small. In the case of the bound stopper formed of a foam material such as urethane foam, it is difficult to determine the maximum stroke of the shock absorber while assuring a suitable buffer clearance (the stroke of the bound stopper abutting on the cylinder of the shock absorber). Thus, the bound stopper in the form of a foam elastic body does not function satisfactorily as a stopper for limiting the stroke of the shock absorber.
In recent years, there is proposed a composite type of bound stopper which consists of two members which are separately formed and generally cylindrical, i.e., a rubber elastic body and a foam elastic body, which are bonded together in the axial direction by means of an instantaneous adhesive agent. The bound stopper of this composite type exhibits a soft spring characteristic based on elastic deformation of the foam elastic body when the stroke is relatively small, and effectively functions as a stopper for limiting the stroke of the shock absorber by use of the rubber elastic body, when the stroke is relatively large.
In the bound stopper of the above composite type, it is difficult to stably ensure a sufficiently high degree of bonding strength between the rubber elastic body and the foam elastic body. Therefore, these two elastic bodies are likely to be separated from each other during transportation of the bound stopper prior to its mounting on the shock absorber, for example, causing difficulties in handling the stopper and some problem in assembling the motor vehicle.
Even if the rubber and foam elastic bodies are secured to each other upon installation of the bound stopper on the motor vehicle, these two elastic bodies, which are repetitively deformed during the use of the stopper, tend to be separated from each other after a short period of use, due to a difference in the load-strain characteristics between the rubber and foam elastic bodies, for example. Since the cylindrical foam elastic body is not supported by any member after separation of the two elastic bodies, there tends to arise noise or vibrations due to abutting contact of the foam elastic body with the rod or piston of the shock absorber. In this condition, the cylindrical foam elastic body may be compressed while it is placed in eccentric relation with the cylindrical rubber elastic body, causing irregular deformation, such as buckling, to these rubber and foam elastic bodies. Thus, the durability of the bound stopper is considerably deteriorated.